Transbay Tube

Transbay Tube

Company type	Infrastructure
Industry	Transportation, Rail transit
Founded	1974
Headquarters	San Francisco, California , United States
Area served	San Francisco Bay Area
Products	Underwater rail passage for Bay Area Rapid Transit (BART)

Transbay Tube

The Transbay Tube is an underwater rail passage that facilitates the transit of Bay Area Rapid Transit's four transbay routes beneath San Francisco Bay, linking San Francisco with Oakland. It connects to a pair of bored tunnels and covers an overall rail distance of roughly between adjacent stations, reaching a maximum depth of below sea level.

Built using the immersed tube technique, the tube was assembled in 57 segments on land, then transported, submerged, and secured underwater primarily through sand and gravel infill.

Inaugurated in 1974 as the concluding segment of the initial BART network, the tube accommodates all BART lines except the Orange Line, making it one of the busiest portions of the system. During peak periods, it transports over 28,000 passengers per hour with intervals as brief as 2.5 minutes. Trains can achieve speeds up to within the tube, generally operating at unless recovering from delays.

Early concepts

The idea for an underwater rail tunnel beneath San Francisco Bay starting from Oakland point was first suggested by San Francisco’s eccentric figure Emperor Norton in 1872. He issued proclamations encouraging Oakland and San Francisco to finance surveys and construction for either a tunnel or bridge to enable rail connectivity.

Formal consideration began in October 1920 under Major General George Washington Goethals, renowned for constructing the Panama Canal. His proposed route—from Market Street to the Oakland Mole—closely resembles today’s Transbay Tube alignment. Goethals envisioned building on bay mud with seismic design considerations, estimating costs up to . His design featured a shield-driven concrete tube with two levels: an upper deck for vehicles and a lower deck for electric trains.

In July 1921, J. Vipond Davies and engineer Ralph Modjeski introduced an alternative bridge-and-tunnel plan near the Southern Crossing, spanning between Mission Rock and Alameda. They emphasized ventilation difficulties for combined vehicle and rail tunnels and supported a dedicated electric rail tunnel instead. Their scheme was among thirteen crossing proposals submitted in 1921, several involving extensive rail tunnels.

In 1947, a joint Army-Navy Commission endorsed an underwater tube to alleviate congestion on the decade-old Bay Bridge. This recommendation followed a feasibility study of the Reber Plan, highlighting the strategic need for an additional transbay crossing.

Construction

Seismic investigations commenced in 1959, followed by boring and testing phases in 1960 and 1964, along with deployment of an earthquake monitoring system on the Bay floor. The tube’s alignment was revised after initial surveys failed to detect continuous bedrock, prompting more detailed probing. The selected route avoids bedrock to allow flexibility and minimize bending stress.

Design plans and alignment were finalized by July 1960. A 1961 estimate placed the Transbay Tube cost at . Construction began in 1965; the last segment was lowered into place on April 3, 1969. To commemorate this event, BART issued bronzed aluminum coins. Prior to interior outfitting, part of the tube was opened to visitors on November 9, 1969. Track installation and electrification concluded in 1973, with service commencing on September 16, 1974—five years behind schedule due to regulatory issues concerning the automated dispatch system. The first automated test run occurred on August 10, 1973; Train No. 222 completed the route at speeds up to 80 mph carrying approximately 100 passengers including officials and media representatives.

The tunnel rests within a trench roughly wide with a gravel foundation about deep. Precision laser-guided dredging maintained route accuracy within for trench dimensions and within for foundation placement. Approximately of material was dredged from the Bay during construction.

The structure comprises 57 sections fabricated at Bethlehem Steel’s Pier 70 shipyard and transported by a large catamaran barge to their final positions. After completing each steel shell, watertight bulkheads were installed followed by pouring concrete walls and track beds approximately -thick. Sections were floated above their designated locations and ballasted with of gravel before being submerged into a trench filled with soft soil, mud, and gravel for leveling purposes. Divers then connected sections underwater, removed bulkheads, and packed protective sand and gravel around them. Cathodic protection systems prevent corrosion caused by saltwater exposure.

The overall project cost about $180 million in 1970 (equivalent to $ in current dollars), with $90 million allocated to construction and the remainder covering rails, electrification, ventilation, and train control infrastructure.

Configuration

The Tube’s western endpoint links directly to the downtown Market Street subway close to the Ferry Building, located north of the Bay Bridge. It passes beneath the western span of the Bay Bridge, connecting the San Francisco Peninsula with Yerba Buena Island, before surfacing in Oakland along 7th Street, to the west of Interstate 880.

Constructed from 57 individual segments, each segment ranges in length from to , averaging along the tunnel's path. These segments measure in width, in height, and weigh approximately apiece. The alignment includes 15 sections curved horizontally, 4 curved vertically, 2 with combined curves, and 36 straight segments. The cost per segment was roughly , according to the original construction contract. The steel shell of the tube is thick, engineered to bear its own weight and withstand hoop stresses. This thin-shell design was verified by engineering consultant Professor Ralph Brazelton Peck, who attributed its stability to soil arching effects.

The structure contains two parallel tunnels separated by a central maintenance and pedestrian gallery. Each tunnel has a bore diameter near , with track centerlines shifted outward from the gallery. The central gallery accommodates maintenance and control systems, including a pressurized firefighting water line. Within each tunnel, there are 56 doors opening into the lower gallery at intervals of about , secured from the gallery side but capable of inward opening during emergencies. Narrow walkways approximately wide run alongside the gallery between these doors.

The upper gallery serves as an air duct channeling ventilation under forced airflow conditions. Ventilation systems are installed at both tunnel termini—San Francisco and Oakland—and between tunnels through remotely controlled dampers measuring by , positioned above every third door.

At both ends, the tube connects to vent structures via patented sliding seismic joints that allow six degrees of freedom—translation and rotation along all three axes—with movement capacities up to axially and vertically or laterally. A restaurant occupies the San Francisco vent structure atop a pier behind the Ferry Building, while the Oakland vent structure is located within a Port of Oakland container terminal.

Seismic retrofitting

Extensive earthquake retrofitting has been performed on both the interior and exterior of the Transbay Tube, with total expenditures estimated at . Following the 1989 Loma Prieta earthquake, a 1991 assessment concluded that seismic joints would likely remain operational; however, ground settling had reduced their permissible movement to as little as .

The 2002 BART Seismic Vulnerability Study highlighted concerns regarding soil liquefaction around the tube that could cause displacement or exceed joint tolerances. To mitigate this risk, compaction of fill material surrounding the tube began in 2006 on property owned by the Port of Oakland. Nevertheless, a 2010 evaluation questioned whether such compaction was necessary given limited uplift potential.

Interior reinforcement efforts commenced in March 2013, involving installation of heavy steel plates designed to strengthen critical sections against lateral seismic forces. Specialized equipment was used to bolt and weld approximately steel plates onto concrete walls. This $7.7 million project was contracted to California Engineering Contractors. The work required early midweek closures of one tunnel bore, resulting in delays of 15–20 minutes but was completed eight months ahead of schedule by December 2013.

In December 2016, BART awarded a $267 million contract for additional seismic improvements including installation of a new steel liner and upgraded high-capacity pumps aimed at reducing flooding risks during significant earthquakes. Scheduled to begin in summer 2018 and extend over two years, this phase will necessitate reduced or suspended service during early morning and late-night hours.

January 1979 fire

On January 17, 1979, an electrical fire broke out aboard a seven-car train (Train No. 117) heading toward San Francisco as it traveled through the Transbay Tube. Lieutenant William Elliott of the Oakland Fire Department tragically lost his life due to inhalation of smoke and toxic gases while combating the blaze. Another train moving in the opposite direction managed to rescue forty passengers along with two BART employees who were on the affected train. Challenges in communication and coordination during this emergency prompted enhancements in safety procedures, ultimately contributing to the formulation of the National Fire Protection Association’s transit guideline NFPA 130, titled Standard for Fixed Guideway Transit and Passenger Rail Systems.

Cause

The fire was triggered by electrical arcing caused by a damaged third rail collector shoe on Train No. 117. Earlier that day, Train No. 363, consisting of ten cars, had made an emergency stop inside the tube due to smoke and suspected fire. Upon inspection, broken derail bars were found on cars six and eight, and car nine’s parking brake was engaged. After these issues were resolved, Train No. 363 resumed operation but inadvertently left a line switchbox cover resting on the third rail. This piece of debris became lodged against the third rail coverboard, damaging the collector shoe on Train No. 117 and resulting in a short circuit.

Subsequent trains initially operated under "road manual" mode, meaning onboard operators controlled them instead of central computer systems. Although debris was observed near where Train No. 363 had stopped, track clearance was declared for service continuation. Train No. 117 was the tenth westbound train to enter the tube after No. 363 and carried 40 passengers when the incident occurred.

Response

At approximately 6:06 p.m., Train No. 117 executed an emergency stop roughly into the Transbay Tube after its operator reported dense smoke. The collector shoe assemblies on cars five and six fractured upon striking a line switchbox cover that had fallen from Train No. 363 and become wedged in the third rail cover. The National Transportation Safety Board (NTSB) determined that electrical arcing ignited components within the car, causing explosions of air tanks and suspension airbags on those cars. Central control deactivated power to the third rail but restored it after 40 seconds in an attempt to uncouple the lead section from the burning cars; this effort was unsuccessful. Ventilation fans were activated at 6:08 p.m. to clear smoke, but power was cut again at 6:15 p.m. A BART line supervisor aboard assisted in gathering passengers in the lead car, including a blind individual.

At 6:09 p.m., BART central dispatch mistakenly contacted San Francisco Fire Department instead of Oakland’s. Oakland firefighters arrived at , where nine firefighters alongside two BART police officers boarded Train No. 900 operating under "road manual." This train halted about into the tube to remove debris and eventually stopped behind Train No. 117. The operator reported damage to the third rail and intense black smoke emanating from a fire at the rear of Train No. 117. Responders became separated; one policeman and seven firefighters entered a gallery between tunnels while others retreated due to smoke conditions. The group in the gallery left westbound tunnel exit door #44 open; meanwhile, another policeman opened door #45 but had to abandon it because of smoke, leaving both doors open.

Train No. 111, carrying over one thousand passengers, had been held at station before moving automatically into the eastbound tunnel at 6:21 p.m., stopping with its rear car near exit door #43 adjacent to Train No. 117. Passengers from Train No. 117 were evacuated through the operator’s window of their lead car into the smoke-filled westbound tunnel, then passed through door #42 into the gallery before boarding Train No. 111 via door #43. Firefighters confirmed all passengers had been evacuated by 6:59 p.m., after which dispatchers ordered Train No. 111 to proceed automatically to West Oakland for hospital transfer.

As Train No. 111 accelerated rapidly to its maximum speed of , pressure differences drew smoke from the westbound tunnel through open doors (#44 and #45) into both the gallery and eastbound tunnel (#43), filling the gallery between doors #43 and #45 with thick smoke. Ten additional firefighters plus a lieutenant entered this gallery via an Oakland ventilation structure wearing portable air masks; they reached between doors #44 and #45 just as Train No. 111 departed, filling their location with dense smoke.

Tunnel exit doors are secured from inside the gallery side requiring keys; however, smoke obscured keyholes preventing firefighters from exiting into the eastbound tunnel.

Firefighters advanced eastward in single file through heavy smoke; near door #51 their air supplies began depleting. Lieutenant William Elliott experienced breathing difficulties and needed assistance. At 7:09 p.m., a firefighter called for help using a phone box; consequently, Train No. 377 was dispatched from West Oakland operating under "road manual" mode for rescue operations. The operator proceeded with passengers onboard without knowledge of ongoing issues.

After returning around 8:10 p.m., firefighters were hospitalized due to exposure effects; Lieutenant Elliott exhausted his oxygen supply and succumbed to smoke inhalation combined with cyanide poisoning.

Aftermath

The response encountered delays partly because of coordination challenges between BART's central dispatch and the fire departments of San Francisco and Oakland. San Francisco reached out to BART at 6:19 p.m., only to be informed that Oakland had already responded. Oakland notified San Francisco about the train fire at 6:32 p.m., leading San Francisco to dispatch units at 6:36 p.m.; however, these units did not begin moving until 7:52 p.m. and arrived on scene by 8:15 p.m., hindered by difficulties in obtaining timely information from both Oakland and BART.

Although the fire was initially reported as contained by 10:45 p.m., it was not completely extinguished until 1:31 a.m. on January 18. Later that day, Oakland firefighters addressed a flare-up within the charred train at BART’s storage facility. BART faced $1.1 million in costs related to repairs inside the tube and safety enhancements, alongside a $1 million loss in revenue due to service interruptions.

Train No. 363 sustained damage across several cars, including missing components and broken derail bars. Train No. 117’s first three cars were affected by smoke damage; its fourth car suffered both smoke and fire damage, while cars five through seven were destroyed entirely. Train No. 900 experienced heat damage to its first car and smoke damage throughout all ten cars. The tunnel itself endured heat-related harm near the fire site, with ceiling concrete spalling that exposed reinforcing steel, overhead cables destroyed, and steel handrails warped.

The shutdown of the third rail power contributed to operational delays, as Train No. 117 was immobilized for forty seconds due to inability to move manually. Both the operator and line supervisor could not uncouple damaged cars because of a short circuit; manual disconnection was necessary but delayed by nine minutes after power was restored, allowing the fire to intensify. Fortunately, low passenger volume combined with a supervisor’s presence enabled rapid evacuation into the lead car. Had Train No. 111 been dispatched carrying over 1,000 passengers, a failure of control cables might have led to a more severe incident.

By February, BART presented revised evacuation strategies to the fire chiefs of San Francisco and Oakland; however, Transbay Tube service did not resume until April 1979. California Public Utilities Commissioner Richard D. Gravelle warned that reopening did not imply guaranteed safety of operations. Both fire departments expressed criticism toward BART officials for not ceding command of the emergency response.

Earthquakes

BART’s emergency procedures mandate trains halt during an earthquake, except for those within the Transbay Tube or Berkeley Hills Tunnel, which continue to the closest station. Following this, lines undergo inspection and service resumes only if no damage is detected.

The most notable incident occurred during the 1989 Loma Prieta earthquake. At that time, a train inside the Transbay Tube was ordered to stop despite the operator not sensing any shaking. Designed with seismic resilience in mind, the tube safeguarded passengers and was cleared for use six hours later, with full system operations restored within twelve hours.

With major highway infrastructure damaged and the Bay Bridge closed for a month due to structural failure, the Transbay Tube became the only direct connection between San Francisco and Oakland. In the week following the earthquake, BART’s daily ridership increased from 218,000 to 308,000 passengers, highlighting its vital role as a transportation lifeline amid regional infrastructure disruptions.

Pedestrians

In both October 2012 and August 2013, unauthorized pedestrian access through the station led to service interruptions and delays within the Transbay Tube. Additionally, in December 2016, an individual remained inside the Tube for over an hour, triggering transit police searches while trains operated at reduced speeds under manual control.

Equipment failure

Aging infrastructure has often caused service interruptions by leaving trains stranded inside the Transbay Tube. Significant events include a train separation due to coupler malfunction in March 2010, a collision between maintenance vehicles in September 2014 that damaged sections of track, and several brake-related stoppages occurring in January 2015, December 2016, and April 2017. These incidents resulted in major delays and necessitated single-track operations during repair periods.

Noise

According to a 2010 survey by the San Francisco Chronicle, the Transbay Tube ranks as the loudest portion of the BART network, with noise levels inside trains reaching approximately 100 decibels—comparable to jackhammer sounds. BART attributes this elevated noise to amplification caused by the concrete tunnel and curved tracks beneath the San Francisco–Oakland Bay Bridge, likening it to banshees or screech owls. After replacing 6,500 feet of rail and grinding three miles in 2015, BART reported diminished noise levels along with favorable feedback from passengers.

Marine traffic

The cathodic protection system’s anodes on the Tube extend from the surrounding trench and face potential damage from vessel anchors. Although anchoring directly above the Tube is prohibited, BART conducts routine inspections to detect any harm to these anodes.

On January 31, 2014, a drifting freighter dropped anchor near the Tube, leading to a brief closure. Following notification from the Coast Guard about the anchor’s proximity, BART suspended service for about 20 minutes to perform inspections; no damage was discovered and operations resumed promptly. During this period, two trains were held back, causing delays of 15 to 20 minutes.

In April 2017, the derrick barge Vengeance, contracted by BART for maintenance on Tube anodes, capsized amid a storm and sank atop the Tube fill. Transit services continued without disruption; primary concerns centered on a diesel spill that was contained within one day.

Future

In 2007, BART unveiled plans projecting growth over the next five decades, anticipating that current tunnel capacity would be reached by 2030. The proposal included constructing a new four-bore Transbay Tube beneath San Francisco Bay, running parallel and south of the existing one. This new tube would surface at the Salesforce Transit Center, facilitating connections with Caltrain and the proposed California High-Speed Rail (CHSR). Designed with two tracks for BART and two for conventional or high-speed rail, this configuration would address differences in rail gauge and safety requirements.

Starting in 2018, BART partnered with CCJPA—the operator of Capitol Corridor commuter rail—to conduct a feasibility study aimed at refining alignment options for this new crossing. The investigation explored standard-gauge railway configurations to enable interoperability among Caltrain, CHSR, Capitol Corridor, and other services.

By September 2022, two routing options were presented: one combining tunnels for both BART and regional rail traffic, and another featuring separate tunnels for each mode.

As of March 2023, due to decreased ridership across the Bay Area, combined four-track or dual-tunnel solutions were considered financially unviable. Consequently, planners shifted toward constructing a single tunnel dedicated exclusively to either BART or regional rail. Link21—the joint planning organization—has been evaluating four potential corridors: two aligned with regional rail expansions and two focused on BART extensions. In November 2024, Link21 recommended adopting standard gauge regional rail over BART’s broader gauge for this tunnel project.

The BART Board endorsed Link21’s recommendation in June 2025, moving forward with development as a standard-gauge rail initiative.

In media

The Transbay Tube served as a filming site during its construction phase for the concluding scene of George Lucas's movie THX 1138. This final vertical ascent sequence was captured inside the unfinished tube prior to the installation of track supports, showcasing Robert Duvall's character climbing on exposed reinforcing rods.

In the television adaptation of Terry Brooks's The Shannara Chronicles, part of the narrative unfolds in the Bay Area, with key characters journeying through the Transbay Tube.

The 2008 video game Dead Space features an audio clip recorded during a trip through the Transbay Tube, enhancing the game's immersive sound environment.

See also

• Key System
• Richmond–San Rafael Bridge: Early concepts by T.A. Tomasini proposed a tunnel from San Francisco connecting to a bridge spanning Albany and Tiburon.

External links

• BART History